DE3883256T2 - DEVICE AND METHOD FOR ATOMIZING LIQUIDS, IN PARTICULAR MELTED METALS. - Google Patents

Abstract

PCT No. PCT/SE88/00671 Sec. 371 Date May 23, 1990 Sec. 102(e) Date May 23, 1990 PCT Filed Dec. 5, 1988 PCT Pub. No. WO89/05197 PCT Pub. Date Jun. 15, 1989.The invention is a method and apparatus for atomizing metal melts by disintegration of a vertical tapping stream of the melt with the aid of horizontal media jets of pressurized gas. The media jets are formed by two slot-shaped nozzles or row of nozzles separate from each other. The jets are oriented to flow at an angle beta between the media jets. A zone is established between the media jets just prior to the intersection of the tapping stream with the media jets. The tapping liquid is drawn back into the zone by the media jets action.

Description

The present invention relates to a method for atomizing liquids, preferably metal melts, by breaking up a preferably vertical jet of liquid by means of preferably horizontal fluid jets of gas or liquid. The invention also relates to a device for carrying out the method.

When liquids are atomized by decomposition using a gas or liquid, extremely small particles are obtained within certain size intervals, which can be considerable.

These known processes can be used for most types of liquids. However, they are primarily used to produce powder from molten metals using a gas, such as nitrogen or argon, as an atomizing medium. Powder produced in this way is often referred to as inert powder and is characterized by its low oxygen content and spherical shape.

Powder metallurgy processes using inertly produced powder encounter various problems regarding the size of the powder particles and/or their size distribution.

Today, finer and/or narrower fractions of inertly prepared powder are desirable for many applications. Such powder is traditionally obtained by classifying a coarser fraction, which gives low yields, or by atomization processes that use extreme gas flows and pressures. Due to its high cost, this powder is used only to a limited extent.

Typical fractions of unclassified powder produced by a number of conventional processes are: 0 - 300 my, 0 - 500 my, 0 - 1000 my. The average particle size in these fractions is 80, 110 and 120 my respectively.

It is difficult to reduce the particle size and the wide range of particle sizes in the finished powder at an acceptable cost.

A number of powder metallurgy (PM) processes are described below, demonstrating the required or preferred powder sizes and fractions that can be achieved by means of the present invention.

PM processes in which products in almost finished form are obtained by hot isostatic pressing without subsequent heat treatment:

Established processes are of limited value today when it comes to achieving high fatigue strength values, since fatigue strength is usually determined by the largest non-metallic inclusions in the material. The impurities arise from the powder production and can only be safely avoided by using a classified fraction whose maximum powder size (= maximum size of the impurities) is not larger than the acceptable defect size. Here desirable Powders can be < 80 um, < 60 um, < 40 um, etc.

Powder for surface coating by welding or spraying:

Certain powders for these purposes are currently produced with yields of less than 50% due to the wide fraction distribution in the manufacturing processes. Typical fractions for these purposes are: 50 to 150 um, 20 to 550 um, 20 to 70 um, 34 to 104 um, etc.

Injection molding is a relatively new process in the field of powder metallurgy:

an extremely fine metal powder fraction is mixed with a plasticizer, the components are then injection molded to extremely tight tolerances. The binder is then burned off in a furnace and the component is subsequently sintered to a high density. Typical desired powder sizes can be: < 15 um, < 22 um, < 44 um, depending on the process used.

Production of alloys that obtain their properties through extremely rapid cooling:

A process for producing a fine powder fraction can in principle be used automatically to produce these alloys, since the by far dominant factor for the cooling rate is inversely proportional to the droplet size.

The process of producing by sintering large products in almost finished form and blanks for further heat treatment such as rolling is an alternative to the more expensive HIP process. The desired size is essentially the same as with injection molding.

Process for producing fiber-reinforced composites with metal matrices. To date, this technique has not been developed on a large scale, but where successful trials have been carried out using PM, this technique has been based on extremely fine powder fractions.

The method according to the invention provides a solution to these and other related problems and is characterized in that two streams of a separation fluid with considerable vertical extension and horizontal flow direction are formed by two slot-shaped nozzles or rows of nozzles which are separated from each other and arranged at the same height, the jets being caused to flow at such an angle β in the horizontal plane between the fluid jets that a zone is formed between the fluid jets immediately before their vertical intersection line in which the suction of a stream of the surrounding medium is compensated by backwards flowing separation fluid, and that the tapping jet is allowed to flow downwards in this zone between the fluid jets.

When one or more gas jets of an atomizing fluid hit a tapping jet during the atomization of metal melts, an instability is induced on the surface of the melt in the contact area between the melt and the gas, causing the melt to spread out into thin films. When these films reach a certain thickness, they are split into thread-like pieces due to the surface tension of the melt. These thread-like pieces break up under the influence of the surface tension into a number of pieces which assume a shape with the lowest possible surface tension, i.e. a spherical shape.

These spherical drops solidify extremely quickly into powder particles due to the heat radiation and the convective heat distribution to the gas.

The size of the particles formed is influenced by a number of parameters, of which the surface tension of the melt and the density and speed of the atomizing medium are the most influential parameters. The speed has a quadratic influence.

It is difficult to influence the surface tension or density for a given melt and atomizing medium, so it is easiest to influence the particle size by means of the velocity of the atomizing medium. In most established atomization processes, high velocities are therefore aimed for by means of a high pressure of the atomizing medium and, in the case of gaseous media, by means of a Laval design of the nozzles.

However, the speed of gaseous atomizing media decreases extremely quickly behind the nozzle, so that usually only a small part of the atomization process takes place within the range of maximum speed.

A larger or smaller portion of the melt is broken down into particles in an area further away from the nozzle, where the velocity is considerably lower, in some cases as low as 10% of the maximum velocity. This results in a powder with a wide range between the smallest and largest particles.

With a method and a device according to the invention, the above-mentioned problems can be significantly reduced, since the contact area between the melt and the atomizing medium is increased many times over. This causes the atomizing process to take place within a short area behind the nozzle in which the speed of the atomizing medium is high.

The invention makes use of a flow phenomenon that occurs when two gas or fluid jets meet at a certain angle. It is known that at or immediately before the intersection point of two fluid jets meeting at an angle, a flow phenomenon occurs that dominates the process to a greater or lesser extent depending on the size of the angle. At small angles, for example less than 5º, the injector effect due to the negative pressure immediately before the intersection point is the dominant property, while at larger angles, for example at 120º, a backward flow of medium occurs with respect to the main direction of flow of the fluid jets.

According to the invention, both phenomena are exploited by choosing the angle between the two fluid jets so that such a large backward flow of medium occurs that it is drawn back into the fluid jets within a short distance by the injector effect. As a result, a zone is created in front of the crossing point in which there is no defined direction, but only two eddy currents with a constant exchange between returning and sucked-in medium. By changing the angle, the extent of this zone is increased or decreased. The angle between the fluid jets can be 0-60º, but is preferably 5-20º.

According to the invention, the atomizing nozzle has the shape of two horizontally directed fluid jets which run parallel to each other in the vertical plane and have a considerable vertical extension compared to their width, and which form an angle with each other in the horizontal plane so that the zone described above is formed. The tapping jet flows downwards from above into the vertical zone formed along the entire height of the nozzle, so that the jet is continuously broken down by the atomizing medium passing along it on its way downwards. Fluid jets with a considerable extension in one direction can be achieved by means of slot-shaped nozzles or by means of a number of circular nozzles which are arranged, for example, closely adjacent in a row. Depending on the prevailing pressure and the medium used, the nozzle for the fluid jets can be designed for negative pressure or for supercritical pressure conditions (Laval nozzle). When the melt flow is properly matched to the capacity of the fluid nozzle, atomization occurs along the entire height of the nozzle.

It can be easily determined whether the correct and maximum capacity is being used, since if the melt flow is too low, the melt will already end up on the way down along the height of the nozzle, and if the melt flow is too high, the melt will exit the lower edge of the nozzle without being atomized. The vertical contact area between gas and melt is suitably five to fifty times longer than the diameter of the tapping jet, preferably ten to thirty times longer than this diameter. A nozzle with a height of 100 mm or more works very reliably with a uniform distribution of the amount of atomized melt per unit height for a typical diameter of the tapping jet, for example 6 mm.

In order to maintain a high velocity of the fluid jets within the atomization area, the fluid nozzles described can be supplemented by one or more additional fluid nozzle pairs. These can be arranged on either side of the main jet containing the melt in order to reduce velocity losses.

In order to prevent non-atomized melt from escaping below the fluid jets in the event of excessive melt flow, the nozzle can be provided with an additional fluid jet that forms a base in relation to the two fluid jets described.

The angle between the tapping jet and the fluid jets can vary. The fluid jet can be essentially horizontal, i.e. the angle between the tapping jet and the fluid jet is 90º, but it can be varied within wide limits. The angle can be 45-135º, preferably between 80º and 100º.

If the fluid jets have an outflow direction that deviates from the horizontal, the angle of the vertical zone described above also changes accordingly, so that this zone and the tapping jet are no longer parallel. This effect can be exploited if it is desirable for the fluid jet to cut further or not as far into the fluid jets on its way down the zone. If the fluid jets are directed upwards with respect to the horizontal plane, the tapping jet in the lower part of the atomization area is further away from the intersection point of the fluid jets. If the fluid jets are directed downwards with respect to the horizontal plane, the opposite occurs, i.e. the tapping jet in the lower part of the atomization area moves closer to the intersection point.

By applying this effect, it is possible to adjust the amount of liquid atomized per unit height of the fluid jets by changing the angle of the fluid jets with respect to the horizontal plane.

Another method of achieving this control is to insert a number of smaller nozzles between the fluid nozzles, these smaller nozzles being vertically distributed and acting in the same direction as the fluid nozzles, but having individually controlled flows directed towards the tapping jet. The number of these nozzles is preferably chosen so that they have the same height as the fluid jets when placed one above the other.

By allowing the tapping jet to strike the zones described above as far away as possible from the intersection of the fluid jets and/or by choosing the horizontal angle between the fluid jets to achieve a stronger tendency towards backflow, the point at which the tapping jet strikes the fluid jets can be controlled along the atomization area by adjusting the flows in the various smaller nozzles. When a fluid jet from the smaller nozzles strikes the tapping jet, the tapping jet is deflected towards the intersection of the fluid jets.

A third method of obtaining these control options is to align the fluid jet nozzles at an angle to each other in the vertical plane, the fluid nozzles are then no longer parallel. Changing this angle causes the distance between the nozzle and the intersection point to vary along the height of the atomization area. Depending on whether the angle is chosen so that the distance between the nozzles is greatest at the top or bottom edge, the zone described is The center line of the tapping jet is inclined away from or towards it. This possibility of controlling the inclination of the zone enables the effect described above of allowing the tapping jet to cut further or not so far into the fluid jets.

The nozzles for the atomizing media or fluids may be movable and adjustable in the horizontal plane to facilitate the adjustment of the point of impact of the tapping jet and fluid jets. The entire nozzle arrangement must then be adjusted to achieve the correct point of impact. The entire nozzle arrangement must then be adjustable to achieve the correct point of impact.

Another way to achieve the desired impact point is to arrange small additional nozzles above the fluid nozzles, which are aligned essentially horizontally and whose outflow is directed towards the tapping jet. By surrounding the tapping jet with a plurality of these additional nozzles, which operate from different directions and have individually adjustable flows, the vertical direction of the tapping jet can be influenced and the desired impact point can thus be achieved.

Using the process described above, small particles with very little size variation can be produced.

According to the invention, an additional improvement in the atomization process can be achieved by inserting guides on each side of the jet behind the point of impact where the fluid jets converge to form a jet containing the melt. The height of the guides corresponds to or is greater than the height of the jet. The guides are arranged in such a way that they reduce the lateral expansion of the jet and thus also the loss of speed in the fluid jet.

The guides may be serrated on the rear edge or otherwise designed to direct the beam alternately toward the centerline and straight ahead along the height.

In such a process, the guide on the opposite side is preferably designed so that the control of the jet is phase-shifted. As a result, the fluid jet is wave-shaped in cross-section along its height. The melt film in the jet is influenced by its alternating lateral deflection, partly by increasing the contact area with the gas and partly by increasing the turbulence in the contact area. Both effects promote the atomization process.

The alternating effect of the fluid jets containing the melt can also be achieved by arranging a number of smaller fluid jets in suitably spaced rows on each side of the fluid jet, located at a suitable distance behind the intersection point of the fluid jets, which are directed so that they strike the fluid jet preferably perpendicularly from the side. The smaller nozzles arranged on each side are arranged at such a distance from one another that the desired alternating effect of the fluid jets is achieved.

The invention also relates to a device for carrying out the method. The features characterizing this device are defined in claim 8. The dependent claims 9 to 11 describe further advantageous features.

The atomization system has a closed system, which preferably operates under a certain overpressure, for example 500 mm water column, so that access to air is prevented. The system comprises a preferably horizontal cylindrical chamber. At the end of the chamber there is a mold or tapping vessel. From this mold, molten metal flows downwards into the chamber via a tapping stone. An atomizing nozzle designed to form two horizontal fluid jets which run parallel in the vertical plane and have a considerable vertical extent compared to their width and which enclose an angle with one another in the horizontal plane so that a neutral zone is formed immediately in front of the intersection point of the jets is arranged in the chamber so that the tapping jet impinges on this zone. The particles produced during atomization are sucked into the gas stream and drawn towards the other end of the chamber; before they reach this end of the chamber they solidify into powder by heat radiation and convective heat distribution to the gas. The chamber is preferably provided with an outlet opening in the end piece towards which the gas/powder mixture flows.

The atomising nozzle may be arranged asymmetrically below the centre line of the chamber so that all the powder accompanies the gas through the exit opening and is not deposited at the bottom of the chamber due to the strong prevailing turbulence. An effect similar to that in fluidisers is then achieved, i.e. the gas from the atomising nozzle is deflected and drawn to the bottom, thus preventing powder from accumulating there. Instead the powder is transported to the exit opening. By arranging a number of gas nozzles in the bottom or sides of the atomising chamber, which together form a gas curtain, this deflection effect can be improved. These gas curtain nozzles should be arranged on the inner circumference of the chamber in two axial rows, one row on each side of the vertical plane of symmetry of the chamber, they should be placed at a height above the floor corresponding to a tangential angle at the circumference equal to or greater than the angle of fall of the powder. The outlet of the gas curtain nozzles is designed so that a curtain-like gas jet is formed parallel to the chamber wall with an angular extension such that an area of the chamber wall is covered which is limited by the direction tangentially downwards along the chamber wall and the direction, for example, 30º below the horizontal plane.

Appropriate spacing of the curtain nozzles with a certain overlap creates a gas curtain along the entire floor, which converges towards the exit opening. The exit opening of the chamber is connected by pipes to a cyclone separator where powder and gas are separated. After separation, the gas can be fed to a compressor via a gas cooler to be returned to the atomizing nozzles. The system includes other necessary valves, cooling equipment and controls for adjusting the gas pressure, temperature, various fluid flows, etc.

The method and device according to the invention also make it possible to carry out spray deposition. The gas-particle mixture is sprayed against a matrix or against a starting blank before the particles have solidified, so that a blank of the alloy in question can be built up. The blanks can be built up on stationary or movable matrices. Particles that do not hit the blank form powder and are treated in the same way as described above for powder.

An embodiment of the invention is described in the accompanying drawings.

Fig. 1 shows the entire arrangement,

Fig. 2a shows the flow process seen from the side,

Fig. 2b shows the same process seen from above,

Fig. 2c shows a modification of the angle between the slots,

Fig. 2d shows the same with two nozzles,

Fig. 3a shows a device with additional nozzles,

Fig. 3b shows the same device from above,

Fig. 3c shows a front view with additional nozzles,

Fig. 4a and 4b show a device with guides, seen from the side and from above,

Fig. 4c shows a modification of the guide,

Fig. 5 shows a device with a number of smaller fluid nozzles,

Fig. 6a shows an atomizing device with a number of inclined nozzles,

Fig. 6b shows the same device seen from one end,

Fig. 7 shows a device with spray deposition.

Fig. 1 shows an apparatus according to the invention with an atomizing chamber 1 which forms part of a closed system which is preferably maintained at a certain overpressure, for example 500 mm water column, in order to prevent air from entering. At one end of the chamber 1 a mold or tapping vessel 2 is arranged. The chamber is preferably horizontal and molten metal runs from the mold 2 down into the chamber 1 via a tapping stone. An atomizing nozzle (3 in Fig. 2a) is designed to form two horizontal fluid jets which run parallel to one another in the vertical plane and have a considerable vertical extension compared to their width and form an angle with one another in the horizontal plane so that a neutral zone is formed immediately in front of the intersection point of the jets. This is arranged in the chamber 1 so that the tapping jet 4 hits this point. This atomization produces particles which are sucked with the gas jet to the other end of the chamber where they solidify into powder by radiation and convection before hitting the end wall of the chamber. The outlet opening of the chamber 1 in the end wall 5 is connected to a cyclone separator 6 in which gas and powder are separated. After separation, the gas flows via a gas cooler 8 to a compressor 7 to be returned to the atomizing nozzle 3.

Figures 2a and 2b show an atomizing nozzle in the form of two horizontally directed fluid jets 9, 10 which run parallel in the vertical plane and have a considerable vertical extension compared to their width. The angle β between the fluid jets has a value such that a zone 11 is formed in which the inflow of the surrounding medium is substantially compensated by the backward outflow of the fluid. The tapping jet 12 is allowed to flow through this zone 11. The angle (G) between the tapping jet and the fluid jets can vary. The fluid jet can be substantially horizontal, i.e. 90º, but the angle can vary between 45º and 135º, preferably it is between 80º and 100º.

The vertical contact area between gas and melt has a length that is suitably 5 to 50 times the diameter of the tapping jet 12, preferably this length is between 10 and 30 times the diameter.

The slot-shaped nozzles 3 can form an angle of 0º, ie they can be parallel, or they can form an acute angle (α) of less than 45º. This is illustrated in Figures 2c and 2d, which show discharge nozzles formed from slot-shaped nozzles and individual nozzles respectively. Changing this angle enables adjusting the inclination of the zone so that the tapping jet cuts deeper or not as deeply into the intersection point of the fluid jets.

The amount of liquid atomized per unit height of the fluid jets can be controlled by angle changes of this kind.

Another device and method with this control possibility has a number of vertically distributed smaller nozzles 13 between the fluid nozzles, these smaller nozzles being directed in the same direction as the fluid nozzles, but having individually controllable flows directed towards the tapping jet. Their total height can be substantially the same as that of the slot nozzles. This is particularly clearly shown in Fig. 3c.

A further improvement in the atomization process can be achieved, as already described above, by using guides 14 (see Fig. 4a and 4b) behind the point of impact 11. These are arranged on each side of the jet, have the same or a slightly greater height than that of the jet and are arranged so as to reduce the transverse expansion of the jet, as shown in Fig. 4b.

The guides may be serrated on the rear edge (see Fig. 4c) or otherwise designed so that the beam is directed along the height alternately towards the centre and straight ahead (15). The resulting effect is described in detail above.

Fig. 5 shows a number of fluid jets 16, which are arranged on each side of the fluid jet at a suitable distance and influence it alternately.

The atomizing nozzle can be arranged asymmetrically (16) below the center line of the chamber 18 so that the powder accompanies the gas through the outlet opening and does not settle on the bottom of the chamber due to the strong prevailing turbulence (see Fig. 6a and 6b). As described above, the gas from the nozzle is then deflected and drawn to the bottom, preventing the powder from accumulating there. The effect can be improved by arranging a number of gas nozzles 17 in the bottom of the chamber, which form a gas curtain. See also the relevant description above.

The method and arrangement according to the invention also enables spray deposition. This means that the gas-particle mixture is sprayed against a matrix 19 (Fig. 7) or against a starting blank before the particles have solidified, so that a blank of the alloy in question is built up. Powder not adhering to the matrix can be collected and used for other purposes, for example as described above.

The devices and methods described above can be varied in many ways within the scope of the claims.

Claims (11)

1. Method for atomizing liquids, preferably metal melts, by breaking up a preferably vertical tapping jet of the liquid by means of preferably horizontal fluid jets of gas or liquid, characterized in that two streams of a decomposition fluid with considerable vertical extension and horizontal flow direction are formed by two slot-shaped nozzles or rows of nozzles which are separated from one another and arranged at the same height, wherein the jets are caused to flow at such an angle β in the horizontal plane between the fluid jets that a zone is formed between the fluid jets immediately in front of their vertical intersection line in which the suction of a stream of the surrounding medium is compensated by reverse-flowing decomposition fluid, and that the tapping jet is allowed to flow downwards in this zone between the fluid jets. 2. Method according to claim 1, characterized in that the slot-shaped nozzles or rows of nozzles run parallel to one another, i.e. the angle between them is zero. 3. Method according to claim 1, characterized in that the slot-shaped nozzles or rows of nozzles are directed at an acute angle to one another, i.e. the angle between them is greater than zero. 4. Method according to at least one of the preceding claims, characterized in that the vertical contact area between gas/liquid and Melt has a length that is 5 to 50 times, preferably 10 to 30 times larger than the diameter of the tapping jet. 5. Method according to at least one of the preceding claims, characterized in that the angle β between the fluid jets is selected between 0 and 60°, preferably between 5 and 20°. E. Method according to one or more of the preceding claims, characterized in that the exit direction of the fluid jets deviates slightly from a completely horizontal direction and as a result the zone is also slightly modified from the vertical position, whereby the angle between the fluid jets and the tapping jet can vary between 45 and 135°, preferably between 80 and 100°, in order to control the amount of atomized melt per unit length of the zone. 7. Method according to one or more of the preceding claims, characterized in that a horizontal fluid jet or fluid jets are generated by separate nozzles which are arranged between and/or behind the slot-shaped nozzles or rows of nozzles, the fluid jets being directed exactly opposite to the tapping jet in order to influence the degree of engagement of the tapping jet in the fluid jet. 8. Device for atomizing melts using the method according to one or more of the preceding claims, with a container (2) for essentially vertical tapping (12) of molten metal downwards to two essentially horizontally aligned fluid jets (19) of gas or liquid, characterized in that the device (2) has essentially vertical, slot-shaped nozzles (3) or rows of nozzles which are arranged at the same height and have considerable vertical extent and whose outflow directions enclose an acute angle β between them in the horizontal plane, so that a zone (11) is formed between the fluid jets immediately in front of their vertical intersection line, in which the suction of a flow of ambient medium is compensated by backwards flowing out decomposition fluid, and that the tapping jet (12) between the fluid jets is allowed to flow downwards in this zone. 9. Device according to claim 8, characterized in that the two slot-shaped nozzles are arranged parallel to one another in a vertical plane or at an acute angle α to one another, i.e. the angle α between them is greater than zero. 10. Device according to claim 8 or 9, characterized in that a further nozzle (13) or nozzles are directed horizontally against the tapping jet, between and/or behind the slot-shaped nozzles or rows of nozzles. 11. Device according to claims 8 to 10, characterized in that a matrix (19) or starting substance is arranged in such a way that the gas/particle mixture is sprayed against it before the particles have solidified.